Many precise industrial processes require machinery in which workpieces, process tools, measurement tools, and the like are accurately positioned and moved, usually while embodying a high degree of automation. An example category of such machinery includes various microlithography systems widely used in the semiconductor-device and micro-electronics industries for transferring images from a pattern-defining reticle onto a semiconductor wafer or other suitable substrate during semiconductor processing. In modern microlithography tools, the need to achieve extraordinarily accurate positioning and movements is critical, requiring these tools to achieve position and motion accuracies of their stages in the nanometer range.
A typical microlithographic exposure apparatus includes an illumination source, a reticle-stage assembly that retains a reticle (or pattern master), an optical assembly, a wafer-stage assembly that retains the substrate, a measurement system, and a control system. The wafer-stage assembly includes a wafer stage that retains the lithographic substrate (such as a semiconductor wafer), and a respective mover assembly that precisely positions the wafer stage and the wafer. Somewhat similarly, the reticle-stage assembly includes a reticle stage that retains the reticle, and a respective mover assembly that positions the reticle stage and the reticle. The control system independently directs current to these mover assemblies to generate forces causing motion of the wafer stage and reticle stage along respective “trajectories.”
The sizes of the images and features within the images transferred onto the substrate (termed generally a “wafer” herein) from the reticle are extremely small. Accordingly, precise positioning of the wafer and the reticle relative to the optical assembly is critical to the manufacture of high-density semiconductor devices. Typically, multiple identical microcircuits are formed on each wafer. Hence, during manufacture of the microcircuits, the wafer stage and/or the reticle stage are cyclically and repetitiously moved to follow an intended trajectory.
During movements of the stages, respective “following-errors” of the wafer stage and/or the reticle stage can occur. A following-error is the difference between the intended trajectory of the stage and an actual trajectory of the stage at a specified time. A following-error can arise due to lack of complete rigidity in the components of the microlithography tool, which is manifest as a slight time delay between the instant in which current is directed to the mover assembly and the instant in which the stage exhibits the corresponding motion.
Alignment errors can occur even if the stages are properly positioned relative to each other. For example, periodic vibration disturbances of various mechanical structures of the microlithography tool can occur. Examples include oscillations and/or resonances of the optical assembly or supporting structures. These oscillations and/or resonances can significantly degrade relative alignment between the stages and the optical assembly. As a result of following-errors and/or the vibration disturbances, the achievable precision with which micro-devices can be manufactured is compromised.
Conventional approaches to reducing following-errors include feedback control of stage motion. In a stage system under feedback control, during movement of one of the stages a measurement system periodically provides data on the current position of the stage. This data is utilized by a controller to adjust the level of current to the mover assembly of the stage in an attempt to achieve the intended trajectory of the stage. Unfortunately, feedback control is not entirely satisfactory, and the control system does not always precisely move the stage along its intended trajectory.
Also, the movable portion of a stage inherently has mass, usually substantial mass. Regardless of applicable tolerances, controlling positions and motions of a stage involves dealing not only with disturbances originating outside the stage but also with disturbances originating in motions (including accelerations and decelerations) of the stage mass itself. No control system is perfect; each has limitations such as following-error and/or synchrony of relative stage motions. The goal of control systems used with these stages is to achieve a level of stage position and motion control sufficient to meet extremely demanding specifications. As specifications progressively tighten, the need for more accurate and precise control follows apace.
In light of the above, there is a need for control systems and methods that improve the accuracy and precision of stage positioning and movement.